A variety of tools and machines have been proposed and/or are in use for tensioning a loop of strap around an article or articles, such as a stack of lumber, equipment on a pallet, and the like. Many kinds of such machines and tools also typically apply a metal seal to secure the overlapping strap lengths together and then sever a trailing portion of the strap length from a supply of the strap on a reel.
Typically, conventional strapping machines and tools of this type grip or hold a leading, free end segment of the strap with a suitable gripping device and then apply tension with a traction wheel which is rotated against the strap. After sufficiently high tension has been pulled on the strap, the tension is maintained on the strap while an open, generally U-shaped seal, which has been supplied from a magazine, is crimped about the overlapping strap portions to hold them together in tight engagement. At the termination of the crimping step, the trailing portion of the strap is severed by a suitable mechanism.
Conventional strapping tools of the type described above have been marketed in the U.S.A. by Signode Corporation, 3600 West Lake Avenue, Glenview, Ill. 60025. One such tool is marketed under the designation "SIGNODE Model ASD Combination Strapping Tool" and is disclosed in the "OPERATION AND PARTS MANUAL" for that tool as published by Signode Corporation under the document designation "REB 7/77-1M-A". Another such machine is marketed under the designation "SIGNODE AM COMBINATION STRAPPING TOOL" and is disclosed in the "OPERATION, PARTS AND SAFETY MANUAL" for that tool as published by Signode Corporation under the document designation "186027 (p. 20E) Rev. 10-89". Other tools of this general type have been marketed under the designation "SIGNODE ASL and ASM COMBINATION STRAPPING TOOLS" and are disclosed in the "OPERATION, PARTS AND SAFETY MANUAL" for such tools as published by Signode Corporation under the document designation "186101 (p. 69A) Rev. 2-90".
The above-identified tools are manually operated and typically include a housing, a tensioning assembly, a seal magazine assembly, a sealer assembly for applying the seal to the overlapping lengths of the strap after the strap has been tensioned, and a cutter mechanism for severing the sealed loop from the trailing portion of strap. Other tools performing the same functions may be pneumatically or electrically operated. Further, the functions may also be incorporated in large, automatic machines which also operate to initially feed the strap around the article to be bound and form a loop which is subsequently tensioned, sealed, and severed from the supply of strap.
The above-identified types of tools and machines typically employ a pair of pivoting jaws for crimping the seal about the overlapping lengths of strap. Typically, the strap and the seal are steel, and the jaws pivot to a closed position to deform the steel seal tightly about the overlapping strap lengths without effecting significant deleterious deformation of the steel strap per se.
Such a conventional jaw mechanism is usually employed in conjunction with a "chair" or anvil. The exterior surface of the crown of the seal is disposed adjacent the anvil with the seal open legs projecting downwardly on either side of the overlapping lengths of strap. The jaws pivot to the closed position and squeeze the seal legs inwardly and upwardly against the strap lengths while the anvil bears the reaction force. In some designs, the anvil is fixed relative to the jaw mechanism, and in other designs, the anvil is moved downwardly a small amount as the jaws close. In some cases, the anvil functions as, or is replaced by, a notching means to notch the edges of the seal and strap to provide increased holding strength.
While conventional sealing mechanisms have generally functioned well in the applications for which they were designed, there is a need for improved performance with respect to some applications to accommodate a variety of strap materials and thicknesses, different seal designs and materials, and different tension levels. An improved sealer mechanism or assembly would be especially desirable for use with metal seals applied to thermoplastic strap.
Conventional tools of the type described above have been used to apply metal seals to plastic strap, but the results, insofar as they are currently known to the present inventor, are not altogether satisfactory. In particular, when a metal seal is applied with conventional sealing mechanisms to overlapping thermoplastic strap, the legs of the seal do not bend and deform to the desired configuration that can be obtained when the same seal is crimped about metal strap.
Further, the thermoplastic strap becomes distorted, deformed, and tends to crack. The overall strength of the clamping effect of the seal is reduced, and the resulting configuration of the seal and strap lengths has protrusions which provide a potential for snagging.
FIG. 1 illustrates a metal seal S1 which has been applied with a conventional sealer mechanism (not illustrated) to overlapping lengths of thermoplastic strap--upper strap length U and lower strap length L. As best illustrated in FIG. 2, the seal S1 has a crown C1 and a pair of legs LG1. The legs LG1 are bent inwardly against the lower strap length L, but the legs LG1 are not parallel to the crown C1. Further, the legs LG1 have caused the upper strap length U and the lower strap length L to buckle downwardly so that there is a void region V1 between the upper strap length U and the seal crown C1 and so that there is another void region V2 between the upper strap length U and the lower strap length L.
It is apparent that there is relatively little surface contact between the seal crown cap C1 and the upper strap length U. Similarly, there is relatively little surface contact between the upper strap length U and the lower strap length L. The seal S1 and the strap lengths U and L are in surface-to-surface contact primarily only at the lateral edges. Thus, whatever joint strength is provided by the crimped seal configuration is provided in spite of this reduced surface-to-surface contact.
In addition, it has been found that the thermoplastic, lower strap length L sometimes tends to crack along the bulging region B where the lower strap length L bulges downwardly between the legs LG1. This cracking can further reduce the strength of the joint and lead to failure of the joint and/or strap under sufficiently high tension loads.
A desired sealed joint configuration for thermoplastic strap as well as metal strap is illustrated in FIG. 3 for a seal S2 having legs LG2 which have been crimped further upwardly so that they are substantially parallel to the strap length U and L and to the seal crown C2. This configuration is typically produced when a metal seal is properly crimped by conventional tools on a metal strap. The inside surface of the seal crown C2 is in surface-to-surface contact with the upper surface of the upper strap length U, the lower surface of the upper strap length U is in surface-to-surface contact with the lower strap length L, and the lower surface of the lower strap length L is in surface-to-surface contact with the seal legs LG2. The substantial surface contact provides increased frictional engagement and increases the strength of the sealed joint.
The strap lengths U and L remain generally flat and do not bulge outwardly. The generally flat configuration of the strap lengths U and L within the seal S2 reduces the potential for cracking and for increased stress concentration regions. Thus, it would be desirable to provide an improved sealer mechanism for producing a sealed joint having a preferred configuration as described above with reference to FIG. 3, and it would be desirable to provide such a sealer mechanism that could be employed to form such a sealed joint with a steel seal on thermoplastic strap as well as on metal strap.
It would also be beneficial to provide an improved sealer mechanism which would have the capability for being adjusted to accommodate a variety of seal designs and sizes as well as a variety of seal materials. Further, it would be desirable to provide such an improved sealer mechanism with the capability for being adjusted to accommodate a variety of strap thicknesses, widths, and materials.